1. Field of the Invention
This invention relates to a method of making multilayer printed wiring boards and more particularly, to a method of making multilayer printed wiring boards in which internal plates each having a conductive circuit formed on its one or both surfaces are laminated via a sheet-like prepreg.
2. Description of the Prior Art
A method of making a multilayer printed wiring board of the prior art is shown in FIGS. 1 and 2, in which a four-layer board is illustrated for the sake of simplification of explanations. As shown in FIG. 1, first, a conductive layer 12 is patterned on the both surfaces of a plate-like insulating plate to form a conductive circuit, thus obtaining an internal plate 11. Each of the conductive layers 12 of the internal plate 11 has a clearance 13a or a portion where the conductive layer 12 is partially removed by patterning. The clearance 13a are partially provided so as to be densely aggregated to form a clearance aggregation section 13.
Next, on each of the both surfaces of the internal plate 11 is laminatedly placed a copper foil 15 via a prepreg sheet 14. The laminated body thus formed of the internal plate 11, prepreg sheet 14 and copper foil 15 is placed in a mold, and heated under the application of a pressure to have them integrally adhered. Thus, a multilayer printed wiring board 10 as shown in FIG. 2 is obtained.
The multilayer printed wiring board 10 thus obtained has recess portions A in the copper foil 15 respectively just above and under the clearance aggregation sections 13. This is because the prepreg sheet 14 goes into the clearance 13a of the conductive layer 12 when heated under the application of a pressure thereby to form a recess portion in the prepreg sheet 14 itself as to be similar to the recess portion A. The recess portion A is also formed in the other sections than the clearance aggregation section 13, however the depth of them to be formed in the former is small as compared with that formed in the latter. As a result, the multilayer printed wiring board 10 thus obtained will become unstable in thickness characteristic and at the same time, the spatial distance between the conductive layer 12 and copper foil 15 will become non-uniform, which means that such a problem is arisen that a through-hole may be disconnected in the plating process as well as a packaged electronic part may be dispersed in electric property.
Thus, an object of this invention is to provide a method of making a multilayer printed wiring board by which a multilayer printed wiring board having a uniform thickness and flat external surfaces as well as having a constant space between the adjacent conductive layers can be obtained even if a patterned conductive layer of an internal plate has a clearance.